Pumps for incompressible fluids, such as oil, are often either gear pumps or vane pumps. In environments such as automotive engine lubrication systems, these pumps will operate over a wide range of speeds, as the engine operating speed changes, resulting in the output volume and the output pressure, as the output of these pumps is generally supplied to a lubrication system which can be modeled as a fixed size orifice, of the pumps changing with their operating speed.
Generally, an engine requires the lubrication oil pressure to increase from a minimum necessary level to a maximum necessary pressure level as the engine operating speed increases, but the maximum necessary oil pressure is generally obtained from the pumps well before the engine reaches its maximum operating speed. Thus, the pumps will provide an oversupply of lubrication oil over a significant portion of the engine operating speed range.
To control this oversupply, and the resulting over pressure which could otherwise damage engine components, constant displacement pumps in such environments are typically provided with a pressure relief valve which allows the undesired portion of the oversupplied oil to return to an oil sump or tank or back to the inlet port of the pump so that only the desired volume, and hence pressure, of fluid is supplied to the engine.
While equipping constant displacement pumps with such pressure relief valves does manage the problems of oversupply at higher operating speeds, there are disadvantages with such systems. For example, the pump is still consuming input energy to pump the oversupply of fluid, even though the pressure relief valve prevents delivery of the undesired portion of the oversupplied fluid, and thus the pump is consuming more engine power than is necessary.
An alternative to constant displacement pumps in such environments is the variable displacement pump, which can be a gear pump or, more commonly a vane pump. Such pumps include a movable control feature, such as the pump ring in vane pumps, which allows the displacement capacity per revolution of the pump to be changed. Typically a control piston, connected to the control feature, is supplied with pressurized oil, directly or indirectly, from the output of the pump and, when the force created by the pressure of the supplied oil on the control piston is sufficient to overcome the force of a biasing spring, the control feature is moved to reduce the displacement of the pump and thus lower the volume and pressure of the pumped oil to a desired level.
If the supplied pressurized oil is at a pressure less than the desired level, then the force generated at the control piston is less than that generated by the biasing spring and the biasing spring will move the control feature to increase the displacement of the pump. In this manner, the output volume (and hence pressure) of the pump can be adjusted to maintain a selected, equilibrium, value of pressure.
While such variable capacity pumps provide advantages over constant capacity pumps and pressure relief valves, it is desirable in some circumstances to further control the displacement of these pumps relative to the speed of the engine, rather than just relative to the output pressure of the pump, thus allowing a designer to change the desired pressure level and/or flow produced by the pump for engine operations at different speeds. Effective displacement control of the pump based at least partially on the operating speed of the engine can result in an improvement in engine efficiency and/or fuel consumption.
While such displacement control is desired, it is also desired that, in the event of a failure of the displacement control system, the system should failsafe such that the engine or other device being supplied by the pump system does not suffer a catastrophic failure. In particular, as a failure of the lubrication oil system can result in catastrophic failure of the engine, it is desired that any speed-related displacement control system must failsafe to prevent damage to the engine.